Early stringed instrument builders discovered that by adding a helical wrapping around the core wire of strings in the bass portion of the scale of the instrument it was possible to increase the level of the sound output and improve the quality of the tone obtained. The function of the wrapping was to cause the tension or pull of the string for a given frequency of vibration to become larger than it would be without a wrapping. It was a characteristic of these antique strings that the wrappings were wide-spaced and extended the full length of the core wire, hence passing over the bridge and around the tuning pins. Insofar as known, the wrapping of such strings was empirical and without defined parameters.
Modern instruments abandoned the use of wide-spaced turns of wrapping and adjacent turns, i.e., contacting or near-contacting turns, have become the norm. In contemporary practice the wrapping extends only over the greater part of the speaking length of the strings, ending just before it reaches either the agraffe or the bridge.
In modern instruments the effect of the wrapping is controlled by the size and weight of the wrapping wire in relation to the core wire used. Until recently, the size of the wrapping wire was normally calculated in terms of the size necessary to produce for a given frequency a particular value of tension or pull on the string. U.S. Pat. No. 3,523,480 taught that another significant factor in the design of wrapped strings is the tuning of the longitudinal mode of vibration of the string. It was found that the tone quality of the instrument could be significantly improved by careful design of the string so that the frequency of the longitudinal mode of vibration would be tuned to a particular value in relation to the transverse or flexural mode of vibration of the string.
In order to control the frequency of the longitudinal mode of the string vibration, careful control of the wrapping wire size and weight is necessary. The minimum diameter of wrapping wire which is practical to use in a piano is on the order of about 0.01" (0.25 mm) if the material of the wrapping wire is copper. If the wrapping material is copper-covered steel wire the smallest practical size is about 0.006" (0.15 mm). For wrapping material appreciably smaller in diameter than these values it becomes increasingly difficult to wrap the string because of the tendency for the wrapping wire to break. In addition, the tonal quality of the string may be impaired by lack of liveness if very small diameter wrapping wire is used.
It is often desirable in the design of a wrapped string to achieve less loading of the core wire than can be achieved by using even the smallest practical size of wrapping wire. The present invention provides the solution to this problem and makes it possible to reduce the amount of loading on the string in a controlled and reproducible way by varying the spacing between the turns rather than by changing the size of the wrapping wire. It may be noted that U.S. Pat. No. 3,605,544 teaches the provision of wrapped strings wherein the turns of the wrapping wire do not make contact with each other. Specifically, the patent teaches that the space between adjacent turns of the wrapping wire is preferably made as small as possible so long as the adjacent turns do not touch each other in vibration, the spacing being from 0.1-0.3 mm. The stated purpose of the invention is to eliminate noises and loss of energy, both of which are stated to be caused by the frictional contact between adjacent convolutions of the wrapping wire. The patent does not address itself to the problem of providing tuned strings by controlling the spacing of the wrapping wire, or of controlling inharmonicity and string tension. In particular, the present invention solves the problem of providing wound strings for a piano in scale ranges where even the lightest practical conventional wraps are still too heavy for the purpose.